FIG. 1 is a schematic sectional view of a pixel of a front-side CMOS image sensor. In this figure and the following, the different pixel elements are shown with dimensions chosen to make the figures intelligible and are not drawn to scale. The doping levels of the P-type conductivity zones are shown with shades of gray that are all the darker as the doping levels are high.
The image sensor is formed in an active layer 10, usually of P-type conductivity, having a doping level noted P−. The layer 10 is formed on a substrate 12, often of P-type conductivity. The layer 10 may have a thickness between 3 and 6 microns, while the substrate may have a thickness of 780 microns.
A buried layer 14 of N-type conductivity, close to the upper face of the layer 10, forms a photodiode with the layer 10. As shown, the portion of the layer 10 above the zone 14 may have a higher doping level than the layer 10 to provide a passivation of the top interface. The upper face of the layer 10 carries various elements for controlling the pixel, especially a transfer gate TG. These elements and other metal tracks are embedded in a passivation layer 16.
The pixel may be laterally isolated from its neighboring pixels by trench isolators 18, typically including semiconductor oxide, which extend throughout the thickness of the active layer 10. Alternatively, the insulation between the pixels may be achieved by an over-doping (P-type) relative to the layer 10, but such insulation is known to be less effective from both an electrical and an optical point of view. The spacing between the trench isolators 18 defines the size of the pixels.
In the case of a color sensor, color filters 19 are formed on the layer 16 in correspondence with the pixels. The filters 19 usually bear individual collimating lenses 20.
In operation, during an integration phase, the photons absorbed in the active layer, i.e. the region 10 of the photodiode, generate electrons that are stored in the region 14 of the photodiode. At the end of the integration phase, the stored charge is proportional to the amount of light received by the photodiode throughout the duration of the integration phase. After the integration phase, the stored charge is transferred through the transfer gate TG to the control elements.
A recurring problem of this pixel structure is the generation of carriers in the photodiode in the absence of light, causing a so-called dark current. The dark current is not the same for all pixels, or between two integration phases of the same pixel. This phenomenon produces a visible noise in the captured images, which is particularly conspicuous in low light conditions.
The origins of dark current are not well known. An identified source is the presence of defects or impurities in the semiconductor and the various interfaces between the active layer, region 10, and the insulating materials that surround it. The semiconductor material and the insulating material are not structurally equivalent, resulting in “construction” defects at the interfaces. All these defects are electrically active.
The interface defects may be neutralized by degenerating the semiconductor side of the silicon-insulator interface. Such a degeneration may be produced by over-doping the semiconductor side so that it has the same properties as a metal, in which the generation-recombination phenomena are balanced naturally. A perfect degeneration is difficult to achieve, whereby defects remain, but in smaller quantities.
In the case of a P-type active region 10, the electrons generated by interface defects diffuse to the storage region, i.e. region 14. These electrons participate in the dark current of the photodiode. To limit this phenomenon, interfaces that may present a poor surface state are neutralized, such as the interface between the trench isolators 18 and the layer 10. As shown, a P-type layer having a higher doping level than the active layer 10 may line the trench isolators 18. The doping level may be the same, P+, as the substrate. Thus, the generated electrons that can diffuse to the region 14 are less numerous. Despite these measures, a dark current may still remain an issue in front-side image sensors.